Continuously variable transmissions (CVT) are commonly used on a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts and scooters. They comprise a driving pulley connected to a motor, a driven pulley connected to wheels or a track, and a trapezoidal drivebelt transmitting torque between the driving pulley and the driven pulley. A CVT automatically changes the ratio as required by load and speed conditions, providing an increased torque under high loads at low speeds and yet controlling the rotation speed of the motor as the vehicle accelerates. A CVT may be used with all kinds of motors, such as internal combustion engines or electric motors.
The sides of the drivebelt are, on each pulley, gripped between two opposite flanges that are coaxially mounted around a main shaft. In each pulley of a conventional CVT, one flange, called xe2x80x9cfirst flangexe2x80x9d, is rigidly connected to one end of the shaft. The other flange, called xe2x80x9csecond flangexe2x80x9d, is free to move with reference to the first flange. At the lowest rotation speed, the winding diameter of the driving pulley is minimal and the winding diameter of the driven pulley is maximum. This is referred to as the minimum ratio since there is the minimum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
When the rotation speed of the driving pulley increases, its second flange moves closer to the first flange thereof under the effect of a centrifugal mechanism. This forces the drivebelt to wind on a larger diameter on the driving pulley and, consequently, on a smaller diameter on the driven pulley. The drivebelt then exerts a radial force on the flanges of the driven pulley in addition to the tangential driving force by which the torque is transmitted. This radial force urges the second flange of the driven pulley away from the first flange thereof. It is counterbalanced in part by a return force generated by a spring inside the driven pulley. It is also counterbalanced by a force generated by the axial reaction of the torque applied by the drivebelt on the driven pulley. This is caused by a cam system that tends to move the second flange towards the first flange as the torque increases. The cam system comprises a cam plate having a plurality of symmetrically-disposed and inclined cam surfaces on which respective cam followers are engaged. The cam followers are usually slider buttons or rollers. The cam plate or the set of cam followers is mounted at the back side of the second flange and the other of them is rigidly connected to the shaft. The closing effect of the cam system on the drivebelt tension is then somewhat proportional to output torque.
At the maximum rotation speed, the ratio is maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each full rotation of the driving pulley.
When the rotation speed of the motor decreases, the winding diameter of the driving pulley decreases and the radial force exerted by the drivebelt decreases as well, allowing the driven pulley to have a larger winding diameter as the spring moves the second flange towards the first flange. There is then a decrease of the ratio.
Ideally, the drivebelt tension is high under high loads at low speeds to prevent the drivebelt from slipping. However, it should be lower at high speeds to avoid excessive pressure on the drivebelt and to maintain a good efficiency. However, to simplify the construction of the driven pulley or because of physical limitations, the spring is set so that the return force is essentially proportional to its deflection, which is in turn proportional to the distance between the first flange and the second flange. In other words, the minimum return force is generated when the first and second flanges are close to each other, and the maximum return force is generated when there is the maximum distance between the two flanges. This is the opposite of the ideal situation since the gripping force on the drivebelt should be maximum at the minimum ratio and minimum at the maximum ratio. A high gripping force at the minimum ratio is particularly important.
Yet, conventional driven pulleys are not well adapted for reverse torque conditions, which are defined as instances during which the torque is transmitted from the driven pulley to the driving pulley. This occurs generally when the vehicle is decelerating or traveling down a hill. The reverse torque tends to move the cam followers away from the cam surfaces. Only the spring counterbalances the torque at that moment but whenever the torque is larger than a given value, the engagement between the cam followers and the cam surfaces may be lost, resulting in an improper ratio. Also, the gripping force of the drivebelt between the flanges decreases in function of the intensity of the reverse torque.
The object of the present invention is to provide an improved driven pulley which allows a control of the return force generated by the spring, particularly for being able to have a large gripping force at the minimum ratio and having a lower one at the maximum ratio. It is also an object of the present invention to provide a driven pulley which maintains a good gripping force on the drivebelt even when subjected to a reverse torque.
More particularly, the present invention provides a driven pulley for use in a continuously variable transmission. The driven pulley is coaxially mountable around a main shaft and comprises a first flange having a conical wall on one side thereof. It also comprises a second flange, coaxial with the first flange, and having opposite first and second sides. The first side is provided with a conical wall facing the conical wall of the first flange to form a drivebelt-receiving groove in which a drivebelt is to be partially wound. The second flange is at least axially movable with reference to the first flange.
The driven pulley is characterized in that it further comprises a first radially-extending support coaxial with the first and the second flange. The first support is at a fixed axial distance from the first flange and facing the second side of the second flange. The driven pulley also comprises at least two inclined first cam surfaces that are substantially identical and symmetrically-disposed on one among the second side of the second flange and the first support. First cam followers are symmetrically connected to other one among the second side of the second flange and the first support. Each first cam follower being engageable with a respective one of the first cam surfaces.
At least two inclined and substantially identical second cam surfaces are symmetrically-disposed on the second side of the second flange. The second cam surfaces have an inverted inclination with reference to the first cam surfaces. A second radially-extending support is further provided. The second support is coaxial with the first and the second flange and is at a fixed axial distance from the first flange.
Second cam followers are symmetrically connected to the second support. Each second cam follower is engageable with a respective one of the second cam surfaces. A biasing element is mounted between the first support and the second support. Finally, the driven pulley also comprises means for connecting the first support to the main shaft in a torque-transmitting engagement.
The driven pulley of the present invention may be used in a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts and scooters. Other applications are possible as well.
A non-restrictive description of the preferred embodiments will now be given with reference to the appended figures.